Osmotic pressure refers to the pressure exerted by dissolved material in a solution on a semipermeable membrane that separates the solution from another solution or from a pure solute such as water. Such a membrane permits water to pass but is impermeable to dissolved substances. Water passes in an attempt to establish a chemical equilibrium between the two sides of the membrane. This passage of water is known as osmosis.
For a dilute solution, osmotic pressure is directly proportional to solute concentration as given by the Van't Hoff equation: EQU .pi.=CRT
Where
.pi.=osmotic pressure PA1 C=solute concentration PA1 R=the gas constant PA1 T=the absolute temperature.
At 25.degree. C., the product RT=24.4 liter.atm.mole .sup.-1. Thus for a 1 molar solution, x=24.4 atm. It can readily be seen that the determination of osmotic pressure (.pi.), affords a convenient method for monitoring the solute concentration and hence the free water available in a biological system.
Osmotic concentration is generally expressed in osmoles. A solution which has an osmotic pressure of 22.4 standard atmospheres is said to have an effective osmotic concentration of one osmole per liter. For any individual substance, the osmole is defined as the weight in grams which gives rise to an osmotic pressure of 22.4 standard atmospheres when dissolved in one liter of solution.
Osmotic measurements can be used for a variety of purposes including molecular weight determination, monitoring physiological parameters, such as the amount of free water present in blood, urine and other biological fluids, determining the moisture content of a gas or a nourished liquid, as well as other industrial applications.
However, state of the art systems for determining the osmolarity of physiological solutions are unsuitable for routine clinical investigations in which rapid screening of large numbers of samples is usually required. Thus, osmotic pressure of physiological fluids such as blood, is generally determined indirectly, based on freezing point depression. This change in state can destroy red blood cells and thereby add soluble material to the plasma, changing the osmolar character of the blood and leading to inaccurate determinations.
The osmometer described in U.S. Pat. No. 3,479,864, although better suited for measuring physiological parameters, also suffers from several shortcomings. This osmometer contains a bed of beads in the measuring loop. Red blood cells can collect and become trapped on the beads and make cleaning difficult.
Current methods for determining the moisture content of a gas, i.e. to protect computers or other scientific equipment from excess moisture, also suffer from several drawbacks. These methods generally utilize a single point of information system. Thus, only the moisture content of a discrete area is determined which can be erroneously high or low.
Osmotic pressure determination also finds use in monitoring the moisture content of solids such as soils. State of the art methods for measuring soil moisture generally require that a dirt sample be removed from the ground, weighed, dried, weighed again and the wet weight compared to the dry weight. Alternatively, liquid can be sucked out of the sample and the volume determined. These methods, however, are time consuming, prone to error and do not discriminate between the presence of fresh water or salty water. Thus, a soil sample may be wet but not have any free water available for the plant.
Another method for determining soil moisture involves the use of a tensiometer. A tensiometer measures the energy status, or potential, of soil water. Tensiometers, however, suffer from several drawbacks. These devices operate in only a small fraction of the water potential range that is normally considered to be available for plant growth. Additionally, the tensiometer only measures water potential in a small volume of soil immediately surrounding the instrument. Further, tensiometers are expensive, fragile and difficult to install. Finally, root fibers can become entangled in these instruments and impede water potential measurement.
U.S. patent application Ser. No. 920,440, now, U.S. Pat. No. 4,805,343 incorporated herein by reference, discloses the use of a hollow fiber system as an osmotic valve for demand watering of plants. Such systems overcome many of the disadvantages of the prior art. It would therefore be desirable to utilize a hollow fiber system in other osmotic applications.